Pressure sensors are commonly used to monitor and/or control the pressure and flow of process fluids such as, for example, oil, water, gases, etc. In many cases, the pressure sensors are integral to a fluid flow regulator that is serially interposed in the process fluid flow path. In other cases, the pressure sensors are integral to a monitoring device that does not perform a regulation function and that is appended to or serially interposed in the process fluid flow path. For example, the RegFlo™ family of flow monitoring devices provided by Fisher Controls International, Inc. (and in particular the RF100 series flow monitoring devices) may include an inlet pressure sensor, an outlet pressure sensor and an auxiliary pressure sensor, all of which may be integrally attached via a threaded engagement.
For some industrial applications such as, for example, the distribution of combustible process fluids (e.g., combustible gases), explosion proof or flame proof rated regulators, sensing devices, etc. are required. In the case of a regulator, combustible process fluids that accumulate in the regulator housing must be properly vented to ambient or atmosphere to achieve a flame proof or explosion proof rating. Generally, atmospheric venting is configured to prevent the dangerous accumulation of combustible process fluids within the regulator housing and to prevent a combustion process that initiates within the regulator housing from propagating to the greater process ambient surrounding the regulator. For example, one or more sintered metal flame arrestors (which are semi-porous and provide a sufficiently minimal flame path) may be used to provide the necessary atmospheric venting.
Known explosion proof and flame proof flow regulators, monitors, etc. are typically configured to use one or more absolute pressure sensors that do not require venting to atmosphere. As is known, absolute pressure sensors usually include a diaphragm that deflects in proportion to the pressure difference across opposing faces of the diaphragm. In the case of an absolute pressure sensor, a reference face or side of the diaphragm is exposed to a sealed chamber while the other side or face of the diaphragm is exposed to the pressure exerted by the process fluid being measured. Because the reference face or side of an absolute pressure sensor diaphragm does not have to be vented to atmosphere, it is a relatively simple matter to attach such a sensor to a regulator or monitor housing without compromising the explosion proof or flame proof rating of the regulator or monitor.
Unfortunately, absolute pressure sensors are not ideally suited for use in applications that require the measurement of low pressure process fluids such as, for example, natural gas distribution applications. In particular, because the reference side or face of the diaphragm is sealed at about atmospheric pressure, absolute pressure sensors are typically only capable of accurately sensing pressures that are well above one atmosphere (i.e., about 15 pounds per square inch absolute).
On the other hand, gauge pressure sensors are better suited to accurately measure relatively low pressures. However, the reference face or side of the diaphragm within a gauge pressure sensor must be vented to atmosphere. As a result, it is not a simple matter to substitute gauge pressure sensors for the absolute pressure sensors that are typically used with explosion proof or flame proof regulators and monitors without compromising the explosion proof or flame proof rating of these devices.
In addition, gauge pressure sensors are typically vented to atmosphere via a small opening in the sensor housing that is exposed to the ambient surrounding the sensor and the regulator or monitor with which it is associated. As a result, gauge pressure sensors typically cannot be used in regulator and/or monitor applications that result in submersion of the regulator and/or monitor in a liquid (e.g., water). For example, many underground applications may involve periodic or continuous submersion of a regulator (and the devices such as sensors and the like attached thereto) in water.